The invention relates to a transmission system for transmitting a multicarrier signal from a transmitter to a receiver.
The invention further relates to a receiver for receiving a multicarrier signal from a transmitter, an interference absorption circuit for detecting interference components included in a multicarrier signal and for substantially removing the interference components from the multicarrier signal and to a method of detecting interference components included in a multicarrier signal and of substantially removing the interference components from the multicarrier signal.
A transmission system according to the preamble is known from the ETSI Standard EN 300 744 V1.1.2 entitled xe2x80x9cDigital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial televisionxe2x80x9d. Multicarrier transmission systems, such as systems employing Orthogonal Frequency Domain Multiplexing (OFDM), enable the transmission of data at a relatively high rate. OFDM, also known as Multicarrier Modulation (MCM) or Discrete MultiTone modulation (DMT), is a technique by which data is transmitted by modulating several low bit rate carriers in parallel, rather than one high bit rate carrier. OFDM is spectrally efficient, and has been shown to be effective for, for example, high performance digital radio links (such as in the Digital Audio Broadcast system). OFDM is also being used in Digital Subscriber Line systems, e.g. ADSL- or HDSL-systems, and in the DVB-T digital terrestrial television broadcast system as described in the above mentioned document.
Such transmission systems are vulnerable to parasitic signal components or interference components which enter the transmission system and which interfere with the desired multicarrier signal. For instance, in the case of the DVB-T transmission system, such interference components, e.g. impulsive noise or ingress noise components, can lead to a full or partial breakdown of the television picture and audio.
An object of the invention is to provide a transmission system for transmitting a multicarrier signal from a transmitter to a receiver, which transmission system is relatively insensitive to interference components. This object is achieved in the transmission system according to the invention, which is characterized in that the receiver comprises an interference absorption circuit for detecting interference components included in the multicarrier signal and for substantially removing the interference components from the multicarrier signal. The known transmission system doesn""t have any provisions for detecting and removing interference components included in the multicarrier signal. By means of such an interference absorption circuit the undesired interference components can be detected and removed from the desired multicarrier signal.
An embodiment of the transmission system according to the invention is characterized in that the interference absorption circuit comprises a circuit input for receiving the multicarrier signal, interference detection means coupled to the circuit input for detecting interference components included in the multicarrier signal and interference removal means coupled to the circuit input for substantially removing the interference components from the multicarrier signal, an output of the interference detection means being coupled to an input of the interference removal means. The main functions of the interference absorption circuit, i.e. detection and removal of interference components, are efficiently implemented by the interference detection means and the interference removal means. The interference detection means and the interference removal means are coupled to each other so that the interference removal means can remove those interference components that have been detected by the interference detection means.
An embodiment of the transmission system according to the invention is characterized in that the interference detection means are arranged for generating and supplying to the output an interference presence signal indicative of the presence of the interference components in the multicarrier signal, the interference removal means being arranged for substantially removing the interference components from the multicarrier signal in dependence on the interference presence signal received at the input. By means of the interference presence signal which indicates the presence of the interference components in the multicarrier signal an effective coupling between the interference detection means and the interference removal means is achieved.
An embodiment of the transmission system according to the invention is characterized in that the interference detection means is arranged for generating the interference presence signal in dependence on the duration of the interference components. As, by this measure, the interference presence signal comprises information related to the duration of the individual interference components the interference removal means can accurately remove the interference components from the signal.
An embodiment of the transmission system according to the invention is characterized in that the interference detection means comprise timing means for generating the interference presence signal. By this measure the interference presence signal can be generated in an efficient and cost effective way.
An embodiment of the transmission system according to the invention is characterized in that the interference detection means further comprise an interference detector coupled to the timing means for detecting the interference components in the multicarrier signal, the timing means comprising a multiple triggerable pulse timer, the interference detector being arranged for generating and supplying to the timing means a number of trigger pulses, the number of trigger pulses being dependent on the duration of the interference components. By these measures a simple, efficient and cost-effective implementation of the interference detection means is obtained. Simple implementations of such an interference detector have been found based upon amplitude detection or slope detection. By triggering the multiple triggerable pulse timer a number of times in dependence on the duration of the individual interference components, the interference presence signal comprises information related to the duration of the individual interference components. Furthermore, multiple triggerable pulse timers are simple and inexpensive and are even available as standard electronic components.
An embodiment of the transmission system according to the invention is characterized in that the interference absorption circuit comprises a desensitizer for temporarily disabling the interference detection means and/or the interference removal means when a repetition rate of the interference components is too high. In this way multiple triggering is prevented under noisy conditions. Under these conditions a relatively large part of the multicarrier signal is removed by the interference removal means, leaving relatively little multicarrier signal content, which might cause an AGC in the receiver to increase the amplification of the multicarrier signal, which in turn leads to more interference components being detected and more parts of the multicarrier signal being removed, etc.
An embodiment of the transmission system according to the invention is characterized in that the interference detector comprise an amplitude detector for detecting the interference components in the multicarrier signal. An amplitude detector has proven to be a simple and effective implementation of an interference detector.
An embodiment of the transmission system according to the invention is characterized in that the amplitude detector comprises an Analog-to-Digital converter. Parts of the interference absorption circuit, e.g. the timing means and the interference removal means, can be implemented effectively in the digital domain, i.e. in digital hardware or as a program on a digital signal processor. In such a digital implementation the overflow signal of the Analog-to-Digital converter can be used to trigger the timing means.
An embodiment of the transmission system according to the invention is characterized in that the interference absorption circuit further comprises delay means for delaying the multicarrier signal, the interference removal means being coupled to the circuit input via the delay means, the delay introduced by the delay means being substantially equal to the delay introduced by the interference detection means. By this measure, any delay introduced by the interference detection means is compensated so that the interference presence signal is in phase with the multicarrier signal. Otherwise, i.e. if the interference presence were out of phase with the multicarrier signal, the removal of the interference components could not be accurately performed.